gitk: rename zh_CN.po to zh_cn.po

When running make from a clean environment, all of the *.po files should
be converted into *.msg files. After that, when make is run without any
changes, make should not do anything.

After beffae7 (gitk: Add Chinese (zh_CN) translation, 2017-03-11),
zh_CN.po was introduced. When make was run, a zh_cn.msg file was
generated (notice the lowercase). However, since make is case-sensitive,
it expects zh_CN.po to generate a zh_CN.msg file so make will keep
reattempting to generate a zh_CN.msg so successive make invocations
result in

    Generating catalog po/zh_cn.msg
    msgfmt --statistics --tcl po/zh_cn.po -l zh_cn -d po/
    317 translated messages.

happening continuously.

Rename zh_CN.po to zh_cn.po so that when make generates the zh_cn.msg
file, it will realize that it was successfully generated and only run
once.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>